1. Technical Field of the Present Invention
This present invention relates to improvements in critical point dryers for sample preparation for electron microscopy and semiconductor wafer manufacturing and especially to a computer controlled critical point drying apparatus. The present invention is embodied in a method for controlling a critical point drying apparatus using a computer system, a computer system for implementing the method of controlling the critical point drying apparatus, and a computer program product bearing software instructions that implement the method of controlling the critical point drying apparatus.
2. Description of the Related Art
The following references provide useful background information on the indicated topics, all of which relate to the present invention, and are incorporated herein by reference:
U.S. Pat. No. 4,055,904 issued to Home on Nov. 1, 1977 describes an automatic method of operating the purge and bleed modes for a critical point dryer.
U.S. Pat. No. 4,104,808 issued to Home et al. on Aug. 8, 1978 describes a critical point dryer wherein the purge and bleed modes are controlled semi automatically.
There will now be provided a discussion of various topics to provide a proper foundation for understanding the present invention.
In order to examine biological specimens under a scanning electron microscope, the biological specimens must be completely dried and coated with a thin conductive layer. It is important that the drying process be accomplished without disturbing the microstructure of the biological specimen to be examined. Depending upon the biological specimen's structure, three techniques are available for drying the biological specimen. The first method is air drying by evaporation of the cellular water. While suitable for bacteria or other rigid structures, this method is detrimental to the structures of many biological specimens. The surface tension forces, which turn a grape into a raisin during the drying process, cause sufficient distortion in the cell structure of many biological specimens thereby rendering them useless. The second method is sublimation or freeze-drying. This method is useful only for very small specimens. Additionally, unless the lengthy technique is followed precisely, structural damage from thermal expansion or ice crystal formation often results. The third method utilized is the phase transitional or critical point drying which produces consistently reproducible results without the drawbacks of the preceding two methods.
Along with being used to prepare specimens for the scanning electron microscope, critical point drying may also be used in the production of MEMS (Micro-Electro-Mechanical Systems) devices. The critical point drying process helps for a sticktion free release of microstructers in the MEMS device.
In critical point drying, a dehydrating fluid such as ethanol or acetone gradually replaces the water contained in a specimen. This maintains the three-dimensional hydrated morphology of the structure under study. However, if the ethanol or acetone evaporates, surface tension forces would cause structural damage and destroy the specimen's usefulness.
Critical point drying devices for sample preparation in electron microscopy are known in the art. The prior art critical point dryers utilize the technique of substituting a transitional fluid for the dehydrating fluid in the cell structure and then removing the transitional fluid. A critical point dryer heats and pressurizes the biological specimen until above the critical pressure and critical temperature. The critical temperature is defined as the temperature above which a gas cannot be liquefied by pressure alone. The critical pressure is the pressure that results when a substance exists as a gas and a liquid in equilibrium at the critical temperature. The critical point of a liquid is when its temperature and pressure are at or above the critical temperature and pressure and the densities of the liquid phase and vapor phase are identical. This critical point is characterized by an absence of phase boundaries that normally exist between a liquid and its vapor at temperatures and pressures below the critical point. This absence of a phase boundary eliminates the boundary forces that exist when changing a liquid to a gas. These boundary forces often cause the destruction of the extremely delicate specimens when changing its internal liquid to a gas below the critical point. Therefore, the solution which is applied in a critical point drying process is to remove the internal liquid from the biological specimen above its critical pressure and temperature to eliminate the boundary force destruction that would otherwise result.
Although all fluids have a characteristic critical point which should allow direct removal without the use of dehydrating or transitional fluids, the critical point temperature and pressure of water is 374.2° C. and 218 atmospheres. Achieving these temperatures and pressures would cause severe damage to most biological specimens and therefore a fluid having a lower critical temperature and pressure is normally substituted. Commonly, a dehydrating fluid is used that is miscible with water (e.g., ethanol or acetone) as an intermediate stage between the specimen containing water and a specimen containing transitional fluid.
Typically, and in the prior art dryers, the transitional fluid commonly used is carbon dioxide (CO2) because it is easy to use, more economical, less noxious and provides consistently better results than other transitional fluids. The critical temperature and pressure of carbon dioxide is 31° C. and 1,072 psi, respectively, thus reducing the potential for destruction of the specimen structure.
The known instruments and apparatuses for critical point drying of biological specimens include, of course, a drying chamber that is connected a supply of the transitional fluid with various regulating valves, temperature gauges and a means for heating the chamber. A skilled technician must carefully control the application, heating, pressurizing and removal of the transitional fluid, thus requiring not only time but also constant attention. Applicants are unaware of a computer-controlled critical point drying apparatus that eliminates the need for constant operator attention.